"Jsou ty dívky studentkami?"

Translation: Are those girls students?

[Veleski00]

Why is the instrumental used here?

[sanquii]

Instrumental can be used for roles or professions, especially if they're temporary.

[vor23]

See this also:
https://www.duolingo.com/comment/25250834

[cmetzner]

would "Are those students girls?" be translated to Czech differently?

[BoneheadBass]

I expect that it would; possibly, for example, as "Jsou ti studenti dívky?"

[VladaFu]

That is indeed a possible sentence, if we have no specific information about them.